1. Field of the Invention
This invention relates to an apparatus and process for pulverization to fine powder material of solids and mixtures of solids, such as polymers, copolymers, homopolymers, rubbers, wood, agricultural products, and mixtures of synthetic and natural polymers which, until now, have been resistant to such fine pulverization at high output.
2. Description of Related Art
Currently, there are three basic reclaiming processes of virgin and used plastics in wide use: chemical reclaiming processes, which include pyrolysis, hydrolysis, and incineration; thermal reclaiming processes, which, for example, include extrusion, injection molding, and pressure molding; and mechanical reclaiming processes, which include granulation, densification, agglomeration, and pulverization. However, these known processes generally have disadvantages ranging from high energy consumption, a reduction in original properties of the polymers, applicability to only specific polymers, and environmental undesirability.
U.S. Pat. No. 4,090,670 teaches recovery of rubber from scrap vulcanized rubber tires by raising the surface temperature sufficiently to devulcanize the rubber tires followed by removal of the devulcanized material, such as by rasping. However, this method is limited to rubber and does not produce fine powders as desired for many reuse applications.
Reclamation of thermoplastic materials including shredding, grinding and comminuting is exemplified by U.S. Pat. No. 4,968,463 which teaches shredding waste plastic to about one hundred millimeters and grinding to under about 40 millimeters, followed by drying, pre-heating to 80xc2x0 C. to 160xc2x0 C., kneading at 120xc2x0 C. to 250xc2x0 C., and injection molding or extruding; U.S. Pat. No. 4,650,126 which teaches heating plastic particles to melt the surface to retain a grinding aid thereon and maintaining a counter-rotating attrition mill at a temperature to retain nearly all of the grinding aid in the softened polymer particles during grinding, followed by an air stream which serves to separate the grinding aid and as a material carrier medium; U.S. Pat. No. 4,511,091 which teaches thermoplastic scrap recovery combined with phonographic record pressing in which the hot trimmed waste is cooled, ground, and mixed with virgin material for formation of phonograph records; and U.S. Pat. No. 4,098,463 which teaches a liquid cooling spray to maintain the temperature in a cutting chamber such that the plastic is hard, which reduces the fibers embedded in the comminuted particles from plastic electrical or telephone cord insulation.
Various screw devices are known for conveyance and processing in the synthetic polymer industry. Molding of products from a mixture of thermoplastic polymers or a thermoplastic polymer and an inorganic material by control of crystallization in a screw extruder with temperature control in a first portion within 35xc2x0 C. below the material melting point and the temperature in a second portion within 35xc2x0 C. above the material melting point, with the maximum temperature at the outlet, is taught by U.S. Pat. No. 5,026,512. U.S. Pat. No. 4,890,996 teaches continuous granulating by melting, kneading and granulating macromolecules wherein a double screw kneader without lateral communication is capable of adjusting the degree to which material is kneaded by axial adjustment of the cylinders and screws with respect to each other.
Conical screw sections are known to be used for specific purposes. A twin screw extruder, especially suited for viscous material, having frusto-conical screw sections and separate barrel sections at the outlet end providing bearing-type support for the separate screws is taught by U.S. Pat. No. 4,875,847. U.S. Pat. No. 3,525,124 teaches an extracting apparatus having screw-threaded shafts rotatable within a housing and having conveying and milling sections within an obstruction section between for pressure sealing. The screw in the housing may be tapered to form the obstruction section, thereby providing independent heat and pressure control in the conveying and milling sections.
Chemical and physical aspects of transformation of polymeric materials, such as pulverization, under simultaneous high pressure and shear is described in Nikolai S. Enikolopian, xe2x80x9cSome Aspects of Chemistry and Physics of Plastic Flowxe2x80x9d, Pure and Applied Chemistry, Vol. 57, No. 11, pp. 1707-1711, (1985).
U.S. Pat. No. 4,607,797 teaches pulverization of used polymers in an extrusion apparatus having a barrel with at least one cylindrical rotatable screw. When two screws are used, they are co-rotational. In accordance with the teachings of this patent, material is fed to one end of the barrel, heated to above its fusing (melting) temperature in a first zone, and cooled to below its solidification temperature with simultaneous pre-crushing and pulverizing of the solidified material in a second zone to form a powdered material which is discharged from the opposite end of the barrel. Screw action is used to convey the material through the barrel and substantially elliptical or triangular kneading or pulverizing disks non-rotatably mounted on the screw in the cooling zone perform the pre-crushing and pulverizing. The process is carried out at 0.25 to 0.30 MPa. This process is said to continuously produce particles having a very uniform grain size, for example, in the case of polyethylene, only 2% larger than 160 microns.
U.S. Pat. No. 4,607,797 teaches pulverization of rubber and vulcanization products in a standard single-or-multiple screw extruder by compressing the material to be pulverized at a pressure of 0.2 to 0.7 MPa and then subjecting the compressed material to a shear force of 0.03 to 5 N/mm2 at a pressure of 0.2 to 50 MPa and a temperature of 80xc2x0 to 250xc2x0 C., forming hot sheared material which is subjected to a shearing force of 0.03 to 5 N/mm2 at a pressure of 0.2 to 50 MPa and a temperature of 15xc2x0 to 60xc2x0 C., forming cooled powdered material. Addition of granulated polyethylene to butyl rubbers is necessary to obtain finely dispersed powders. This process is said to result in particles not exceeding 500 micrometers in the case of natural rubber and 300 micrometers with other rubbers.
Natural and synthetic polymer wastes are increasing and environmental concerns about their disposition render recycling necessary. However, many of the aforementioned reclamation processes are limited to certain types of wastes and particularly limited with respect to mixed wastes, are uneconomical, particularly with respect to energy consumption, and do not provide reclaimed material in a form conducive to reuse manufacturing.
These issues are addressed by the extrusion pulverization apparatus and method taught by U.S. Pat. No. 5,397,065 and U.S. Pat. No. 5,704,555, in which the polymeric material is heated to a softening temperature below its melting temperature to produce a continuous film of polymeric material which is then cooled and subjected to shear and normal forces sufficient to form the film of polymeric material into a fine powder material. A gas stream then fluidizes the fine powder material to prevent agglomeration and/or melting of the fine powder material.
See also U.S. Pat. No. 5,743,471, which teaches an apparatus having at least one screw, housed within an elongated barrel, having a high shear screw section for applying shear and normal forces to polymeric material to form a powder of polymeric material.
U.S. Pat. No. 5,769,355 teaches an apparatus and method for pulverizing solid polymer materials into smaller particles wherein the apparatus has a pulverizer head with a rotor having a conical contact surface and a stationary dish with a corresponding inverted conical contact surface. The dish and the rotor are axially aligned and spaced apart to form a gap. Rotation of the rotor generates shear forces within the gap that pulverizes the material, with the pulverized particles leaving the pulverizer head at the outer margin of the gap. Thus, no pulverization occurs within a housing.
Although the known extrusion pulverization methods and apparatuses address many of the issues associated with prior pulverization methods and apparatuses, there remains significant room for improvement.
It is one object of this invention to provide an apparatus and process for pulverization of polymeric material having increased powder throughput compared to known apparatuses and processes utilizing an extruder.
It is another object of this invention to provide a process and apparatus for pulverization of polymeric materials which produce finer powder material particles than known processes and apparatuses utilizing an extruder.
It is yet another object of this invention to provide an apparatus and process for pulverization of polymeric material which substantially prevent agglomeration of the fine powder material produced by the pulverization of the polymeric material.
It is yet another object of this invention to provide an extrusion pulverization process and apparatus which provide high shear force on the material, independent of the extruder driver.
These and other objects of this invention are addressed by an apparatus for extrusion pulverization of a polymeric material comprising a multi-component screw having an extrusion portion and a pulverization portion with a controllable shear force, said extrusion portion and said pulverization portion connected to each other and being independently rotatable with respect to each other. In accordance with one preferred embodiment, a plurality of multi-component screws, each having an extrusion portion and a pulverization portion, are disposed within the elongated cylindrical housing.
In the process for extrusion pulverization of polymeric materials according to this invention, polymeric material is fed into the extruder feeding section of an elongated cylindrical housing in which is disposed the extrusion portion of a multi-component screw which rotates to move the polymeric material through the extrusion section of the elongated cylindrical housing. Depending on the physical nature and properties of the polymeric material, the polymeric material can be heated in the extrusion section. The material moves through the extrusion section to the pulverization section of the elongated cylindrical housing which is disposed downstream of the extrusion section. Although, in general, no pulverization is expected to occur within the extrusion section, depending on the physical nature of the polymeric material and temperature profile of the elongated cylindrical housing, partial pulverization may occur. Within the pulverization section, the polymeric material passes between the inner surface of the elongated cylindrical housing and the outer surface of the pulverization portion wherein controllable forces are applied to the polymeric material sufficient to form a fine powder. Cooling is employed to maintain the temperature of the pulverized material low enough to prevent agglomeration or melting of produced particles. Additionally, or in the alternative, an inner pulverization screw portion cooling and/or a cooling jacket surrounding the pulverization section of the elongated cylindrical housing may be utilized for this purpose.
The apparatus of this invention is applicable to a variety of polymers, such as, high and low density polyethylene, rubber, mixtures of high and low density polyethylene and rubber, and high and low density polyethylene and wood, copolymers, phenolic resins and thermoset polyurethane. The fine particle polymeric material produced by the apparatus of this invention enables much broader end uses as fillers and reinforcement agents in matrices, such as polymer, ceramic, gypsum, concrete, and asphalt. This is important to practical utilization of products of recycling polymer wastes, both pre- and/or post-consumer, to reduce the environmental problems caused by such solid wastes.